Method for making RIM polyurethane elastomers using as the catalyst system a hydroxyalkylamine, dibutyltin dilaurate and an alkyltin mercaptide

ABSTRACT

The invention is a method for making reaction injection molded polyurethane of improved paintability properties. The product comprises the reaction product of a high molecular weight polyhydric polyether, a low molecular weight active hydrogen containing compound of at least two functionality, a polyisocyanate and a catalyst combination comprising N-hydroxypropyl-N&#39;,N&#39;,N&#34;,N&#34;-tetramethyliminobispropylamine, dibutyltin dilaurate and an alkyltin mercaptide. This catalyst combination results in improved processing, a minimum of surface defects, excellent green strength and improved paintability using certain paints. Reaction injection molded elastomers are useful as molded articles of commerce including, but not limited to, vehicle body parts.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns the field of reaction injection moldedpolyurethanes.

2. Description of the Prior Art

Reaction Injection Molding (RIM) is a technique for the rapid mixing andmolding of large, fast curing urethane parts. RIM polyurethane parts areused in a variety of exterior body applications on automobiles wheretheir light weight contributes to energy conservation. RIM parts aregenerally made by rapidly mixing active hydrogen containing materialswith polyisocyanate and placing the mixture into a mold where reactionproceeds. These active hydrogen containing materials comprise a highmolecular weight polyhydric polyether and a low molecular weight activehydrogen containing compound. After reaction and demolding, the partsmay be subjected to an additional curing step which comprises placingthe parts in an ambient temperature of about 250° F. or greater.

In a previously filed patent application Ser. No. 136,199 filed Apr. 1,1980, now U.S. Pat. No. 4,273,885, a catalyst system for RIMpolyurethane elastomers was described which was comprised ofdimorpholinodiethylether, dibutyltin dilaurate and an alkyltinmercaptide. The catalyst combination in that patent application impartssuperior processing characteristics to RIM polyurethane elastomersystems. However, we have since discovered that the use ofdimorpholinodiethylether, while advantageous in many RIM systems,interferes in the cure of certain important paint systems known as highsolids enamel paints. We have discovered that by substituting a reactiveamine catalyst, which is tied up in the polymer network by reaction, fordimorpholinodiethylether that the processing benefits already describedin the above mentioned patent application are retained and the RIM partcan be painted using the high solids enamel paint systems. The reactiveamine catalyst used in our present invention isN-hydroxypropyl-N',N',N",N"-tetramethyliminobispropylamine. This aminecatalyst is described and claimed in U.S. Pat. No. 4,101,470 to EdwardE. McEntire.

SUMMARY OF THE INVENTION

The invention is a method for making reaction injection moldedpolyurethane of improved processing characteristics and properties. Theproduct comprises the reaction product of a high molecular weightpolyhydric polyether (polyol), a low molecular weight active hydrogencontaining compound of at least 2 functionality, a polyisocyanate and acatalyst combination comprisingN-hydroxypropyl-N',N',N",N"-tetramethyliminobispropylamine, dibutiltindialurate and alkyltin mercaptide. The invention is also the resultingRIM polyurethane composition.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The polyols useful in the RIM elastomers of this invention includepolyether polyols, polyester diols, triols, tetrols, etc., having anequivalent weight of at least 500, and preferably at least 1000 up toabout 3000. Those polyether polyols based on trihydric initiators ofabout 4000 molecular weight and above are especially preferred. Thepolyethers may be prepared from lower alkylene oxides such as ethyleneoxide, propylene oxide, butylene oxide or mixtures of propylene oxide,butylene oxide and/or ethylene oxide. In order to achieve the rapidreaction rates which are normally required for molding RIM polyurethaneelastomers, it is preferable that the polyol be capped with enoughethylene oxide to increase the reaction rate of the polyurethanemixture. Normally at least 50% primary hydroxyl is preferred, althoughamounts of primary hydroxyl less than this are acceptable if thereaction rate is rapid enough to be useful in industrial application.Other high molecular weight polyols which may be useful in thisinvention are polyesters or hydroxyl terminated rubbers (such ashydroxyl terminated polybutadiene). Hydroxyl terminatedquasi-prepolymers of polyols and isocyanates are also useful in thisinvention.

The chain-extenders useful in this invention are preferablydifunctional. Mixtures of difunctional and trifunctional chain-extendersare also useful in this invention. The chain-extenders useful in thisinvention include diols, amino alcohols, diamines or mixtures thereof.Low molecular weight linear diols such as 1,4-butanediol and ethyleneglycol have been found suitable for use in this invention. Ethyleneglycol is especially preferred. Other chain-extenders including cyclicdiols such as 1,4-cyclohexane diol and ring containing diols such asbishydroxyethylhydroquinone, amide or ester containing diols or aminoalcohols, aromatic diamines and aliphatic amines would also be suitableas chain-extenders in the practice of this invention.

A wide variety of aromatic polyisocyanates may be used here. Typicalaromatic polyisocyanates include p-phenylene diisocyanate, polymethylenepolyphenylisocyanate, 2,6-toluene diisocyanate, dianisidinediisocyanate, bitolylene diisocyanate, naphthalene-1,4-diisocyanate,bis(4-isocyanatophenyl)methane, bis(3-methyl-3-isocyantophenyl)methane,bis(3-methyl-4-isocyanatophenyl)methane, and 4,4'-diphenylpropanediisocyanate.

Other aromatic polyisocyanates used in the practice of the invention aremethylene-bridged polyphenyl polyisocyanate mixtures which have afunctionality of from about 2 to about 4. These latter isocyanatecompounds are generally produced by the phosgenation of correspondingmethylene bridged polyphenyl polyamines, which are conventionallyproduced by the reaction of formaldehyde and primary aromatic amines,such as aniline, in the presence of hydrochloric acid and/or otheracidic catalysts. Known processes for preparing polyamines andcorresponding methylene-bridged polyphenyl polyisocyanates therefrom aredescribed in the literature and in many patents, for example, U.S. Pat.Nos. 2,683,730; 2,950,263; 3,012,008; 3,344,162 and 3,362,979.

Usually methylene-bridged polyphenyl polyisocyanate mixtures containabout 20 to about 100 weight percent methylene diphenyldiisocyanateisomers, with the remainder being polymethylene polyphenyl diisocyanateshaving higher functionalities and higher molecular weights. Typical ofthese are polyphenyl polyisocyanate mixtures containing about 20 to 100weight percent methylene diphenyldiisocyanate isomers, of which 20 toabout 95 weight percent thereof is the 4,4'-isomer with the remainderbeing polymethylene polyphenyl polyisocyanates of higher molecularweight and functionality that have an average functionality of fromabout 2.1 to about 3.5. These isocyanate mixtures are known,commercially available materials and can be prepared by the processdescribed in U.S. Pat. No. 3,362,979, issued Jan. 9, 1968 to Floyd E.Bentley.

By far the most preferred aromatic polyisocyanate is methylenebis(4-phenylisocyanate) or MDI. Pure MDI, quasi-prepolymers of MDI,modified pure MDI, etc. Materials of this type may be used to preparesuitable RIM elastomers. Since pure MDI is a solid and, thus, ofteninconvenient to use, liquid products based on MDI are often used and areincluded in the scope of the terms MDI or methylenebis(4-phenylisocyanate) used herein. U.S. Pat. No. 3,394,164 is anexample of a liquid MDI product. More generally uretonimine modifiedpure MDI is included also. This product is made by heating puredistilled MDI in the presence of a catalyst. The liquid product is amixture of pure MDI and modified MDI: ##STR1## Examples of commercialmaterials of this type are Upjohn's ISONATE® 125M (pure MDI) andISONATE® 143L ("liquid" MDI). Preferably the amount of isocyanates usedis the stoichiometric amount based on all the ingredients in theformulation or greater than the stoichiometric amount.

It has been found that an improvement in processing characteristics andpaintability using high solids paints of reaction injection molded (RIM)polyurethanes using a combination of ingredients chosen from thoseenumerated above may be had by employment of a particular catalystcombination. This combination comprises generallyN-hydroxypropyl-N',N',N",N"-tetramethyliminobispropylamine, a fastgelation organo tin catalyst and a delayed action gelation organo tincatalyst. A delayed action catalyst begins catalytic activity after aperiod of time has passed after mixing with the reactants. Thisparticular combination of catalyst types results in valuable processingimprovements including excellent flow properties in the mold, a minimumof surface defects due to shrinkage and excellent green strength. Inorder to achieve these results and obtain a RIM part with excellentpaintability characteristics when using high solids paints a particularcatalyst combination has been found to be useful as follows. It ispreferred thatN-hydroxypropyl-N',N',N",N"-tetramethyliminobispropylamine be used asthe weak tertiary amine catalyst. The fast gelation tin catalyst ispreferred to be dibutyltin dilaurate. The delayed action gelation tincatalyst is preferred to be an alkyltin mercaptide. This alkyltinmercaptide may preferably be a commercial product known as FOMREZ®UL-29. Although each individual catalyst in this combination is wellknown to be a catalyst in the urethane art, the particular combinationhere provides the advantages of processing, green strength, and absenceof surface flaws which is absolutely necessary to the successfulcommercialization of RIM parts as well as excellent paintability usinghigh solids paints.

The RIM formulation includes a great number of other recognizedingredients such as additional crosslinkers, catalysts, extenders,blowing agents and the like. Blowing agents may include halogenatedlow-boiling hydrocarbons, such as trichloromonofluoromethane andmethylene chloride, carbon dioxide, nitrogen, etc., used.

Other conventional formulation ingredients may also be employed, suchas, for example, foam stabilizers, also known as silicon oils oremulsifiers. The foam stabilizer may be an organic silane or siloxane.For example, compounds may be used having the formula:

    RSi[O-(R.sub.2 SiO).sub.n -(oxyalkylene).sub.m R].sub.3

wherein R is an alkyl group containing from 1 to 4 carbon atoms; n is aninteger of from 4 to 8; m is an integer of from 20 to 40; and theoxyalkylene groups are derived from propylene oxide and ethylene oxide.See, for example, U.S. Pat. No. 3,194,773.

Although not essential for the practice of this invention, the use ofcommonly known additives which enhance the color or properties of thepolyurethane elastomer may be used as desired. For example, chopped ormilled glass fibers, chopped or milled carbon fibers and/or othermineral fibers are useful.

In a preferred embodiment of this invention, a high molecular weightpolyether polyurethane polyol of about 5000 molecular weight or above iscombined with a stoichiometric excess of 4,4'-diphenylmethanediisocyanate (MDI) and allowed to react in the presence of a catalystcombination ofN-hydroxypropyl-N',N',N",N"-tetramethyliminobispropylamine, dibutyltindilaurate and an alkyltin mercaptide in a standard RIM machine usingknown processing techniques. In an especially preferred embodiment ofthis invention the molded RIM part from just above is post cured at atemperature of about 325° F. for about one half of an hour. Theinvention may be exemplified by the following examples which are notintended to limit the scope of the invention.

A glossary of terms and materials used in the following examples followsthe examples.

EXAMPLE I

THANOL® SF-5505 (16 pbw), ethylene glycol (6.44 pbw), PLURONIC® F-98surfactant (0.22 pbw), Fomrez UL-29 (0.025 pbw), dibutyltin dilaurate(0.015 pbw) andN-hydroxypropyl-N',N',N",N"-tetramethyliminobispropylamine (0.1 pbw)were premixed and charged into the B-component working tank of anAccuratio VR-100 high pressure RIM impingement mixing machine. Isonate143L (29.83 pbw) and THANATE® L55-O (5.78 pbw) were premixed and chargedinto the A-component working tank. The A-component temperature wasadjusted to 80° F. and the B-component temperature was adjusted to 110°F. The impingement mixing pressure of the A-component was adjusted to2300 psi and that of the B-component to 2200 psi. The machine wascalibrated to deliver an A/B weight ratio of 1.569. The materials werethen impingement mixed at the conditions given above and then caused toflow into a steel mold preheated to 170° F. measuring 18"×18"×1/8".Plaques made by the above process were postcured at 325° F. for 30minutes and allowed to equilibrate at ASTM conditions for a one weekperiod prior to testing. The plaques had a room temperature flexuralmodulus of about 85,000 psi.

Immediately after release from the mold, the plaques showed littleevidence of crazing upon bending, indicating excellent "green strength".The plaques also had an excellent overall surface appearance, showing noevidence of surface flaws or "shrink marks". The plaques were tested forpaintability by high solids enamel paint systems and passed thesetests.* When THANCAT® DMDEE is used in place ofN-hydroxypropyl-N',N',N",N"-tetramethyliminodibpropylamine, similarexcellent properties result except that plaques containing THANCAT DMDEEdo not pass the paintability tests when high solids enamel paint systemsare used. Thus,N-hydroxypropyl-N',N',N",N"-tetramethyliminobispropylamine is superiorin this latter test.

PAINT TESTING FOR RIM POLYURETHANE ELASTOMERS

A 4"×12"×1/8" sample of RIM polyurethane is first washed thoroughly toeliminate all the mold release on the surface. Even small amounts ofmold release will interfere with the adhesion of the paint film to thesubstrate. After washing and drying, the samples are then painted. Thepaint systems most important in reinforced RIM are presently theso-called "high solids" enamel paints. When RIM elastomers containingTHANCAT® DMDEE, an unreactive tertiary amine catalyst, are painted withPPG's 430 high solids enamel paint system, interference with paint cureand adhesion failure are observed. WhenN-hydroxypropyl-N',N',N",N"-tetramethyliminobispropylamine issubstituted for THANCAT DMDEE in the RIM formulation, the abovementioned problems do not occur.

GLOSSARY OF TERMS AND MATERIALS

RIM--Reaction Injection Molding

Polyol--A di- or greater functionality high molecular weight alcoholterminated molecule composed of ether groups such as ethylene,propylene, butylene, etc., oxides.

MDI--4,4' diphenyl methane diisocyanate

ISONATE 143L--Pure MDI isocyanate modified so that it is a liquid attemperatures where MDI crystallizes--product of the Upjohn Co.

THANOL SF-5505--a 5500 molecular weight polyether triol containingapproximately 80% primary hydroxyl groups.

THANCAT DMDEE--Dimorpholinodiethylether

FOMREZ UL-29--A stannic diester of a thiol acid (an alkyl tinmercaptide). The exact composition is unknown. Product of Witco ChemicalCo.

PLURONIC F-98--A 13,500 molecular weight polyether diol containing about80% by weight ethylene oxide.

THANATE L55-O--A quasi prepolymer consisting of 50 parts by weight ofISONATE 143L and 50 parts by weight of THANOL SF-5505.

We claim:
 1. In a method for making a polyurethane elastomer ofsignificantly improved properties wherein an aromatic polyisocyanate, apolyol of above about 500 equivalent weight, a chain extending agentcomprising a low molecular weight active hydrogen containing compound ofat least 2 functionality and a catalyst system is injected via a RIMmachine into a mold cavity of the desired configuration the improvementwhich comprisesusing as the catalyst systemN-hydroxypropyl-N',N',N",N"-tetramethyliminobispropylamine, dibutyltindilaurate and an alkyltin mercaptide.
 2. A method as in claim 1 whereinthe polyol comprises a polyether of about 5000 molecular weight based ona trihydric initiator.
 3. A method as in claim 1 wherein thepolyisocyanate comprises 4,4'diphenylmethane diisocyanate.
 4. A methodas in claim 1 wherein the elastomer is postcured at about 325° F.